It has become more and more common in recent years to replace the magnetic tape or disk storage media which in the past were used as bulk digital data storage media with solid state devices, such as for example, charge-coupled devices, as well as metal oxide semiconductor and bipolar solid state memory devices. In many cases, the replacement of magnetic media with solid state devices has posed considerable problems in devising the most efficient ways to use these new media, from both cost and performance standpoints. The situation is further complicated by the fact that it is desirable that solid state memory units designed to replace magnetic units be plug compatible therewith, that is to say, preferably no modifications are required to be made to the host computer unit before the solid state unit can be substituted for a supplanted magnetic storage media. This plug compatability requirement places further constraints on the design of the solid state memory unit. The present invention relates to the provision of address marks which are uniquely coded digital data which are stored along with the information data to which they pertain and which provide an unequivocal indication to a reading unit that the beginning and/or end of a given record of digital data has been reached.
The problem of providing address marks has been treated differently in various prior art memory technologies. For example, in the case of magnetic tape storage media, it is relatively simplified because the tape is sequential, that is, as it can only be unreeled continuously there is no question of saving access time by skipping over intervening data. Therefore, data records stored on tape need only have an address mark at the beginning and end of a given record. Such an address mark has usually been provided by coding together clock pulses (which are reliably detectable in the case of tape because the tape travels at a constant speed) together with particularly selected data pulses. In the case of a magnetic disk record a somewhat different scheme is used, because there the decision must be made as to on which side of which of a number of stacked disks a given record is stored, where along the radius it is stored, and in what sector of the disk it is stored. Given such information, the disk can be subdivided into sufficiently small sections that the address mark need only comprise a single synchronizing pulse per sector, so that it is possible to determine where on the circumference of a given disk the read-write head is at any given time. Again, the constant speed of motion of the disk provides a ready means of synchronizing reading and storage media. As in the case of tape, usually clock pulses together with data bits are encoded at the beginning of a digital data record stored on magnetic disk media; usually the address mark so generated is stored along with count and key data fields, which are used to determine the length and type of the data record. Such clock-pulse synchronous arrangements as discussed above are not applicable to solid state memory media because solid state memory media do not spin or travel at a constant speed and hence cannot be reliably synchronized which is essential if clock-pulses are to be distinguishable from data pulses. In many cases, in fact, there are no clocking means required to operate a solid state memory at all. Therefore, some new means must be found in order to indicate the beginning and/or end of a given data record within a solid state memory media, one which will nevertheless provide a unique and reliable method of detecting the beginning and end of data records.